Internet DRAFT - draft-ietf-netconf-adaptive-subscription
draft-ietf-netconf-adaptive-subscription
NETCONF Working Group Q. Wu
Internet-Draft W. Song
Intended status: Experimental Huawei
Expires: 14 June 2024 P. Liu
China Mobile
Q. Ma
Huawei
W. Wang
China Telecom
Z. Niu
Microsoft
12 December 2023
Adaptive Subscription to YANG Notification
draft-ietf-netconf-adaptive-subscription-04
Abstract
This document defines a YANG data model and associated mechanism that
enable adaptive subscription to a publisher's event streams. The
periodic update interval for the event streams can be set adaptively.
Applying these elements allows servers to automatically adjust the
rate and volume of telemetry traffic sent from a publisher to
receivers.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 14 June 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Model Overview . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Subscription Configuration . . . . . . . . . . . . . . . 6
2.2. YANG RPC . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2.1. "establish-subscription" RPC . . . . . . . . . . . . 9
2.3. Notifications for Adaptive Subscribed Content . . . . . . 10
3. XPath Complexity Evaluation . . . . . . . . . . . . . . . . . 10
4. Adaptive Subscription YANG Module . . . . . . . . . . . . . . 12
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
5.1. Updates to the IETF XML Registry . . . . . . . . . . . . 17
5.2. Updates to the YANG Module Names Registry . . . . . . . . 18
6. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 19
8. Acknowledges . . . . . . . . . . . . . . . . . . . . . . . . 19
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Normative References . . . . . . . . . . . . . . . . . . 19
9.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Example YANG Module . . . . . . . . . . . . . . . . 21
A.1. "example-wifi-network-diagnostic" YANG Module . . . . . . 22
Appendix B. Adaptive Subscription and Notification Example . . . 26
B.1. "edit-config" Example . . . . . . . . . . . . . . . . . . 27
B.2. Create Adaptive Subscription Example . . . . . . . . . . 29
B.3. "xpath-evaluation-unsupported" error response example . . 29
B.4. "adaptive-period-update" notification example . . . . . . 30
Appendix C. Changes between Revisions . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
1. Introduction
YANG-Push subscriptions [RFC8641] allow subscriber applications to
request a continuous customized stream of updates from a YANG
datastore without needing to poll. It defines a mechanism (i.e.,
update trigger) to determine when an update record needs to be
generated. Two types of subscription are introduced in [RFC8641],
distinguished by how updates are triggered: periodic and on-change.
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* Periodic subscription allows subscribed data to be streamed to the
destination at a configured fixed periodic interval;
* On-change subscription allows update to be triggered whenever a
change in the subscribed information is detected.
However, in some large-scale deployments where an increased data
collection rate is used, it becomes more likely that both clients and
servers could be temporarily overwhelmed with a burst of streamed
data, and network and computation resources could be excessively
consumed. Therefore, it may be expensive to continuously monitor
operational data at a high collection rate. Conversely, if the rate
at which we collect a stream of data is set too low or low priority
telemetry data is chosen to be dropped, these telemetry data will not
be sufficient to detect and diagnose problems and to verify correct
network behavior.
There is a need for a service to balance between data management cost
and real-time streaming telemetry. To achieve this, servers can be
configured with multiple different period intervals and corresponding
subscription update policies which allow servers/publishers to
automatically switch to different period intervals according to the
network condition changes without the interaction with the client for
policy update instructions. Take wireless network performance
monitoring as an example, when the wireless signal strength falls
below a configured threshold, the subscribed data can be streamed at
a higher rate to capture potentially important data and events (e.g.,
continuous service degeneration); while when the wireless signal
strength crosses a configured threshold, the subscribed data can be
streamed at a lower rate. Another case to consider is to balance the
impact of continuous streaming updates on the CPU utilization of the
device. When the CPU utilization on the device exceeds a certain
high-level value, it might be desired for the monitoring service to
specify a less rapid period interval to make room to save more
resources. Conversely, When the CPU usage is lower than the
specified threshold, a shorter period interval may be used without
exhausting the CPU resources at the same time.
A client might choose to monitor the operational state and send a
request to modify the data collection rate on the server as needed.
But how often the client evaluates if the modification of the data
collection rate is required highly depends on the current collection
rate, collecting a stream of data at a low rate prevents the
subscriber from capturing sufficient data for timely decision-making.
In addition, when tens of thousands of network devices need to be
managed, frequent follow-up modification requests are prone to
errors.
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This document defines a YANG data model and associated mechanism that
enable adaptive subscription to a publisher's event streams. The
periodic update interval for the event streams can be set adaptively.
Applying these elements allows servers to automatically adjust the
rate and volume of telemetry traffic sent from a publisher to
receivers.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
The following terms are defined in [RFC5277], [RFC7950], [RFC3198],
[RFC8342], [RFC8639], [RFC8641] and are not redefined here:
* Event
* Client
* Configuration
* Configured subscription
* Configuration datastore
* Notification message
* Publisher
* Receiver
* Subscriber
* Subscription
* On-change subscription
* Periodic subscription
* Selection filter
This document defines the following term:
Adaptive Subscription: A subscription that specifies subscription
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period update policy on the servers when the subscription is
initialized and allows servers/publishers to automatically switch
to different period intervals according to network condition
changes without interacting with the client for update policy
instructions.
2. Model Overview
This document defines a YANG data model named "ietf-adapt-
subscription" which augments the "update-trigger" choice defined in
the "ietf-yang-push" module [RFC8641] with subscription configuration
parameters that are specific to a subscriber's adaptive subscription.
In addition to subscription state notifications defined in [RFC8639]
and notifications for subscribed content defined in [RFC8641], "ietf-
adapt-subscription" module also defines "adaptive-period-update"
notification to report the update interval change.
The following tree diagrams [RFC8340] provide an overview of the data
model for "ietf-adapt-subscription" module.
module: ietf-adapt-subscription
augment /sn:subscriptions/sn:subscription/yp:update-trigger:
+--:(adaptive-periodic)
+--rw adaptive-periods
+--rw adaptive-period* [name]
+--rw name yang:yang-identifier
+--rw xpath-eval-criterion yang:xpath1.0
+--rw period yp:centiseconds
+--rw anchor-time? yang:date-and-time
augment /sn:establish-subscription/sn:input/yp:update-trigger:
+--:(adaptive-periodic)
+-- adaptive-periods
+-- adaptive-period* [name]
+-- name yang:yang-identifier
+-- xpath-eval-criterion yang:xpath1.0
+-- period yp:centiseconds
+-- anchor-time? yang:date-and-time
augment /sn:modify-subscription/sn:input/yp:update-trigger:
+--:(adaptive-periodic)
+-- adaptive-periods
+-- adaptive-period* [name]
+-- name yang:yang-identifier
+-- xpath-eval-criterion yang:xpath1.0
+-- period yp:centiseconds
+-- anchor-time? yang:date-and-time
augment /sn:subscription-started/yp:update-trigger:
+--:(adaptive-periodic)
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+-- adaptive-periods
+-- adaptive-period* [name]
+-- name yang:yang-identifier
+-- xpath-eval-criterion yang:xpath1.0
+-- period yp:centiseconds
+-- anchor-time? yang:date-and-time
augment /sn:subscription-modified/yp:update-trigger:
+--:(adaptive-periodic)
+-- adaptive-periods
+-- adaptive-period* [name]
+-- name yang:yang-identifier
+-- xpath-eval-criterion yang:xpath1.0
+-- period yp:centiseconds
+-- anchor-time? yang:date-and-time
notifications:
+---n adaptive-period-update
+--ro id? sn:subscription-id
+--ro period yp:centiseconds
+--ro period-update-time? yang:date-and-time
+--ro datastore identityref
+--ro (selection-filter)?
+--:(by-reference)
| +--ro selection-filter-ref
selection-filter-ref
+--:(within-subscription)
+--ro (filter-spec)?
+--:(datastore-subtree-filter)
| +--ro datastore-subtree-filter?
<anydata> {sn:subtree}?
+--:(datastore-xpath-filter)
+--ro datastore-xpath-filter?
yang:xpath1.0 {sn:xpath}?
2.1. Subscription Configuration
For adaptive subscriptions, triggered updates will occur at the
boundaries of specified time intervals when the corresponding trigger
condition is satisfied. These boundaries can be calculated from the
following adaptive periodic parameters:
* a "name" represents the name of each adaptive period;
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* a "period" defines the duration between push updates, in units of
0.01 seconds. The "period" has the same definition as the
yp:period for periodic subscription defined in [RFC8641], while it
must be present with an "xpath-eval-criterion" parameter so that
update interval can be switched based on trigger conditions
indicated by the "xpath-eval-criterion" parameter;
* an "anchor-time"; update intervals fall on the points in time that
are a multiple of a "period" from an "anchor-time". If an
"anchor-time" is not provided, then the "anchor-time" MUST be set
with the creation time of the initial update record inside each
periodic timeframe.
* an "xpath-eval-criterion" represents a standard XPath evaluation
expression (See section 6.4 of [RFC7950]) that is applied against
the targeted data node, which is used to trigger/control the
update interval switching within the server. It follows the rules
defined in section 3.4 of [XPATH1.0] and contains a comparison of
a targeted node instance to the specific threshold in the XPath
format. Different from selection filter defined in [RFC8641], it
monitors a specific data node instance change and evaluates the
trigger condition associated with the specified threshold value to
be true or false using XPath rules and does not affect the even/
update record output generation from a publisher. The updates are
only pushed as the corresponding period interval when the XPath
expression is evaluated to "true".
It is not always trivial for a subscriber to determine the
threshold used in an XPath expression criterion. Sometimes the
threshold needs to be adjusted during the lifecycle of an adaptive
subscription, depending on historical fluctuation range and how
rapid the targeted value of the node changes, distribution
characteristics of the targeted node value or even the generated
volume of telemetry traffic.
How often the XPath expression criterion is evaluated to decide
whether to switch to another period interval is up to the
publisher's implementation. With minimal delay, the expression
can be evaluated whenever changes to targeted data node occur, or
at the end of each shortest streaming update period. To reduce
the frequency of evaluation, the server can choose to check
targeted node change at every multiple (e.g.,2 or 3) high-
frequency streaming update periods.
The represented expression defined in "xpath-eval-criterion" is
evaluated in the following XPath context:
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- The set of namespace declarations is the set of prefix and
namespace pairs for all YANG modules implemented by the server,
where the prefix is the YANG module name and the namespace is
as defined by the "namespace" statement in the YANG module.
- If the leaf is encoded in XML, all namespace declarations in
scope on the "xpath-eval-criterion" leaf element are added to
the set of namespace declarations. If a prefix found in the
XML is already present in the set of namespace declarations,
the namespace in the XML is used.
- The set of variable bindings is empty.
- The function library is the core function library defined in
[XPATH1.0] and the function defined in Section 10 in RFC 7950.
- The context node is the root node in the accessible tree which
is the operational state data in the server.
For the cases where the "xpath-eval-criterion" parameter refers to
multiple list/leaf-list instances, XPath abbreviated syntax can be
used to identify a particular instance, e.g., to represent a
comparison for a leaf in a list entry:
/if:interfaces/if:interface[if:name="eth0"]/if:in-errors>1000.
The server MUST convert the XPath evaluation expression defined in
"xpath-eval-criterion" to a boolean value and internally apply the
"boolean" function defined in Section 4.3 in [XPATH1.0] if the
evaluated result is not a boolean value. It MUST evaluates to
"false" if the target node instance to be compared is deleted.
Only if the XPath expression is evaluated to "true", does the
publisher switch to the corresponding period with which push
updates are reported.
Note that the adaptive subscription may not be supported by every
YANG datastore node. A publisher MAY decide to simply reject an
adaptive subscription with "adaptive-unsupported" (defined in
Section 2.2.1.1) if the scope of the subscription contains selected
data nodes for which adaptive subscription is not supported.
2.2. YANG RPC
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2.2.1. "establish-subscription" RPC
The augmentation of YANG module "ietf-yang-push" made to RPCs
specified in YANG module "ietf-subscribed-notifications" [RFC8639] is
introduced. This augmentation concerns the "establish-subscription"
RPC, which is augmented with parameters that are needed to specify a
subscriber's adaptive subscriptions. These parameters are the same
as the ones defined in Section 2.1.
2.2.1.1. RPC Failures
As specified in [RFC8639] and [RFC8641], RPC error responses from the
publisher are used to indicate a rejection of an RPC for any reason.
This document introduces three new RPC errors for "establish-
subscription" RPC.
establish-subscription
-----------------------------
adaptive-unsupported
xpath-evaluation-unsupported
multi-xpath-criteria-conflict
The "adaptive-unsupported" RPC error is used to indicate that the
adaptive subscription is not supported for the targeted set of data
nodes that are selected by the filter.
The "xpath-evaluation-unsupported" RPC error is used to indicate that
a server failed to parse syntax defined in "xpath-eval-criterion".
The failure can be caused by either a syntax error or some XPath 1.0
syntax not supported against the specific data node.
When specified, multiple XPath evaluation criteria inside "adaptive-
periods" container MUST be mutually exclusive. The "multi-xpath-
criteria-conflict" error is used to indicate that the multiple XPath
evaluation criteria represented by "xpath-eval-criterion" are
evaluated as conflicting, i.e., more than one condition expressions
are evaluated to "true". As the operational state on the device
changes, multiple XPath evaluation criteria may also conflict with
each other during the lifetime of an adaptive subscription, the
publisher may still push updates at the shortest streaming period
among multiple corresponding period intervals in such cases.
For an example of how the above RPC errors can be returned, see the
"xpath-evaluation-unsupported" error response illustrated in
Appendix B.3.
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Note that existing RPC errors defined in RFC 8639 and RFC 8641 are
still supported by this document. For example, if any configured
period for adaptive subscription is not supported by the publisher, a
"period-unsupported" error response could be used.
2.3. Notifications for Adaptive Subscribed Content
The adaptive update notification is similar to subscription state
change notifications defined in [RFC8639]. It is inserted into the
sequence of notification messages sent to a particular receiver. As
stated in RFC 8639, section 2.7, the adaptive update notification
cannot be dropped or filtered out, it cannot be stored in replay
buffers, and it is delivered only to impacted receivers of a
subscription. The identification of the adaptive update notification
is easy to separate from other notification messages through the use
of the YANG extension "subscription-state-notif". This extension
tags a notification as a subscription state change notification.
The elements in the 'adaptive-period-update' notification include:
* a "period" that defines the duration between push updates, the
period can be changed based on trigger conditions.
* a "period-update-time" that designates a timestamp when the server
starts to switch to another period interval because the evaluated
"xpath-eval-criterion" expression result changed.
* A selection filter to identify YANG nodes of interest in a
datastore. Filter contents are specified via a reference to an
existing filter or via an in-line definition for only that
subscription based on XPath evaluation criteria defined in section
6.4 of [RFC7950]. Referenced filters allow an implementation to
avoid evaluating filter acceptability during a dynamic
subscription request. The "case" statement differentiates the
options. Note that filter contents are not affected by the
"xpath-eval-criterion" parameter defined by the update trigger.
3. XPath Complexity Evaluation
YANG-Push subscriptions [RFC8641] specify selection filters to
identify targeted YANG datastore nodes and/or datastore subtrees for
which updates are to be pushed. In addition, it specifies update
policies which contain conditions that trigger generation and pushing
of new update records. To support a subscriber's adaptive
subscription defined in this document, the trigger condition also
uses similar selection filters to express a standard XPath evaluation
criterion (section 6.4 of [RFC7950]) that is applied against the
targeted data node(s).
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The following complex implementation and use choices need to be
cautious, although they have already been well supported by the
section 3.4 of [XPATH1.0]:
* Support XPath evaluation criteria for all "config true" and
"config false" data nodes;
* Support more than one target data node selection and operation
(e.g., addition, subtraction, division and multiplication) in a
single XPath evaluation criterion;
* Support any type of node value in the XPath evaluation criterion,
e.g., string, int64, uint64, and decimal64 types;
* Both objects in the XPath evaluation criterion to be compared are
node-sets;
* Targeted data to be compared are in different data types, e.g.,
one is an integer, the other is a string.
As described in section 6.4 of [RFC7950], Numbers in XPath 1.0 are
IEEE 754 [IEEE754-2008] double-precision floating-point values; some
values of int64, uint64, and decimal64 types cannot be exactly
represented in XPath expressions.
If targeted data to be compared are in different data types, a
conversion function is needed to convert different data types into
numbers.
If both objects in XPath evaluation criteria to be compared are node-
sets, more computation resources are required which add complexity.
To reduce these complexities, the following implementation and use
principles are recommended:
* XPath evaluation criteria are applied against a minimal set of
data nodes in the data model, the minimal set of data nodes can be
advertised using "ietf-notification-capabilities" module defined
in [RFC9196];
* Both targets in the XPath evaluation criterion to be compared are
in the same data type;
* One target to be compared in the XPath evaluation criterion is a
leaf/leaf-list data node with numerical data type (e.g., signed/
unsigned integer) and the other is a numerical threshold value.
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If a server receives an XPath evaluation criterion with some XPath
syntax unsupported against the specific targeted data node, an RPC
error with "xpath-evaluation-unsupported" MUST be returned.
4. Adaptive Subscription YANG Module
This YANG module makes imports from [RFC8639], [RFC8641] and
[I-D.ietf-netmod-rfc6991-bis].
<CODE BEGINS> file "ietf-adapt-subscription@2023-12-13.yang"
module ietf-adapt-subscription {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-adapt-subscription";
prefix as;
import ietf-subscribed-notifications {
prefix sn;
reference
"RFC 8639: Subscription to YANG Notifications";
}
import ietf-yang-push {
prefix yp;
reference
"RFC 8641: Subscription to YANG Notifications for Datastore
Updates";
}
import ietf-yang-types {
prefix yang;
reference
"RFC YYYY: Common YANG Data Types";
}
organization
"IETF NETCONF (Network Configuration) Working Group";
contact
"WG Web: <http://tools.ietf.org/wg/netconf>
WG List: <netconf@ietf.org>
Editor: Qin Wu
<mailto:bill.wu@huawei.com>
Editor: Wei Song
<mailto: songwei80@huawei.com>
Editor: Peng Liu
<mailto: liupengyjy@chinamobile.com>
Editor: Qiufang Ma
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<mailto: maqiufang1@huawei.com>
Editor: Wei Wang
<mailto: wangw36@chinatelecom.cn>
Editor: Zhixiong Niu
<mailto: Zhixiong.Niu@microsoft.com>";
description
"This module extends the YANG data module defined in
YANG-push to enable the subscriber's adaptive
subscriptions to a publisher's event streams with various
different period intervals to report updates.
Copyright (c) 2023 IETF Trust and the persons identified
as authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with
or without modification, is permitted pursuant to, and
subject to the license terms contained in, the Revised
BSD License set forth in Section 4.c of the IETF Trust's
Legal Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info).
This version of this YANG module is part of RFC xxxx
(https://www.rfc-editor.org/info/rfcxxxx); see the RFC
itself for full legal notices.
The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED',
'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document
are to be interpreted as described in BCP 14 (RFC 2119)
(RFC 8174) when, and only when, they appear in all
capitals, as shown here.";
revision 2023-12-13 {
description
"Initial revision";
reference
"RFC xxxx: Adaptive Subscription to YANG Notification.";
}
identity adaptive-unsupported {
base sn:establish-subscription-error;
description
"Adaptive-subscription is not supported for the targeted set
of objects that are selectable by the filter.";
}
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identity xpath-evaluation-unsupported {
base sn:establish-subscription-error;
description
"Unable to parse the xpath evaluation criteria defined in
'xpath-eval-criterion' because of a syntax error or some
XPath 1.0 syntax not supported against the specific
data node.";
}
identity multi-xpath-criteria-conflict {
base sn:establish-subscription-error;
base sn:subscription-terminated-reason;
description
"Multiple Xpath evaluation criteria represented by
'xpath-eval-criterion' are evaluated as a conflict. I.e.,
more than one condition expression is evaluated to
'true'.";
}
grouping adaptive-subscription-modifiable {
description
"This grouping describes the datastore-specific adaptive
subscription conditions that can be changed during the
lifetime of the subscription.";
container adaptive-periods {
list adaptive-period {
key "name";
description
"An entry in this list represents an adaptive period
which defines a push update interval and trigger
conditions to switch to the update interval for
sending an event record to the subscriber.";
leaf name {
type yang:yang-identifier;
description
"The name of adaptive period.";
}
leaf xpath-eval-criterion {
type yang:xpath1.0;
mandatory true;
description
"An XPath string, representing a logical expression,
which can contain comparisons of datastore values
and logical operations in the XPath format.";
}
leaf period {
type yp:centiseconds;
mandatory true;
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description
"Duration of time that should occur between periodic
push updates, in units of 0.01 seconds.";
}
leaf anchor-time {
type yang:date-and-time;
description
"Designates a timestamp before or after which a series
of periodic push updates is determined. The next
update will take place at a point in time that is a
multiple of a period from the 'anchor-time'.
For example, for an 'anchor-time' that is set for the
top of a particular minute and a period interval of a
minute, updates will be sent at the top of every
minute that this subscription is active.";
}
}
description
"Container for adaptively periodic subscription.";
}
}
augment "/sn:subscriptions/sn:subscription/yp:update-trigger" {
description
"This augmentation adds additional subscription parameters
that apply specifically to adaptive subscription.";
case adaptive-periodic {
description
"Defines adaptively periodic case for sending an event
record to the subscriber.";
uses adaptive-subscription-modifiable;
}
}
augment
"/sn:establish-subscription/sn:input/yp:update-trigger" {
description
"This augmentation adds additional establish-subscription
parameters that apply specifically to datastore updates to
RPC input.";
case adaptive-periodic {
description
"Defines adaptively periodic case for sending an event
record to the subscriber.";
uses adaptive-subscription-modifiable;
}
}
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augment "/sn:modify-subscription/sn:input/yp:update-trigger" {
description
"This augmentation adds additional modify-subscription
parameters that apply specifically to datastore updates to
RPC input.";
case adaptive-periodic {
description
"Defines adaptively periodic case for sending an event
record to the subscriber.";
uses adaptive-subscription-modifiable;
}
}
augment "/sn:subscription-started/yp:update-trigger" {
description
"This augmentation adds additional adaptive subscription
parameters to the notification that a subscription has
started.";
case adaptive-periodic {
description
"Defines adaptively periodic case for sending an event
record to the subscriber.";
uses adaptive-subscription-modifiable;
}
}
augment "/sn:subscription-modified/yp:update-trigger" {
description
"This augmentation adds additional adaptive subscription
parameters to the notification that a subscription has been
modified.";
case adaptive-periodic {
description
"Defines adaptively periodic case for sending an event
record to the subscriber.";
uses adaptive-subscription-modifiable;
}
}
notification adaptive-period-update {
sn:subscription-state-notification;
description
"This notification contains a push update that in turn
contains data subscribed to via a subscription. In the
case of a periodic subscription, this notification is sent
for periodic updates. It can also be used for
synchronization updates of an on-change subscription.
This notification shall only be sent to receivers of a
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subscription. It does not constitute a general-purpose
notification that would be subscribable as part of the
NETCONF event stream by any receiver.";
leaf id {
type sn:subscription-id;
description
"This references the subscription that drove the
notification to be sent.";
}
leaf period {
type yp:centiseconds;
mandatory true;
description
"New duration of time that should occur between periodic
push updates, in units of 0.01 seconds.";
}
leaf period-update-time {
type yang:date-and-time;
description
"Designates a timestamp when the server starts to switch
to another period interval because the evaluated 'xpath-
external-eval' expression result changed.";
}
uses yp:datastore-criteria {
refine "selection-filter/within-subscription" {
description
"Specifies the selection filter and where it originated
from. If the 'selection-filter-ref' is populated, the
filter in the subscription came from the 'filters'
container. Otherwise, it is populated in-line as part
of the subscription itself.";
}
}
}
}
<CODE ENDS>
5. IANA Considerations
5.1. Updates to the IETF XML Registry
This document registers one URI in the IETF XML registry [RFC3688].
Following the format in [RFC3688], the following registration is
requested to be made:
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---------------------------------------------------------------------
URI: urn:ietf:params:xml:ns:yang:ietf-adapt-subscription
Registrant Contact: The IESG.
XML: N/A, the requested URI is an XML namespace.
---------------------------------------------------------------------
5.2. Updates to the YANG Module Names Registry
This document registers one YANG module in the YANG Module Names
registry [RFC7950]. Following the format in [RFC6020], the following
registration is requested to be made:
---------------------------------------------------------------------
Name: ietf-adapt-subscription
Namespace: urn:ietf:params:xml:ns:yang:ietf-adapt-subscription
Prefix: as
Reference: RFC xxxx
---------------------------------------------------------------------
6. Security Considerations
The YANG module specified in this document defines a schema for data
that is designed to be accessed via network management protocols such
as NETCONF [RFC6241] or RESTCONF [RFC8040]. The lowest NETCONF layer
is the secure transport layer, and the mandatory-to-implement secure
transport is Secure Shell (SSH) [RFC6242]. The lowest RESTCONF layer
is HTTPS, and the mandatory-to-implement secure transport is TLS
[RFC8446].
The NETCONF Configuration Access Control Model (NACM) [RFC8341]
provides the means to restrict access for particular NETCONF or
RESTCONF users to a preconfigured subset of all available NETCONF or
RESTCONF protocol operations and content.
There are a number of data nodes defined in this YANG module that are
writable/creatable/deletable (i.e., config true, which is the
default). These data nodes may be considered sensitive in some
network environments. Write operations (e.g., edit-config) to these
data nodes without proper protection can have a negative effect on
network operations. These are the subtrees and data nodes and their
sensitivity/vulnerability:
* /sn:subscriptions/sn:subscription/yp:update-trigger/as:adaptive-
periods/as:adaptive-period/as:period
* /sn:subscriptions/sn:subscription/yp:update-trigger/as:adaptive-
periods/as:adaptive-period/as:anchor-time
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* /sn:establish-subscription/sn:input/yp:update-trigger/as:adaptive-
periods/as:adaptive-period/as:period
* /sn:establish-subscription/sn:input/yp:update-trigger/as:adaptive-
periods/as:adaptive-period/as:anchor-time
7. Contributors
Thanks Michael Wang, Liang Geng for their major contributions to the
initial modeling and use cases.
Michael Wang
Email: wangzitao@huawei.com
Liang Geng
China Mobile
32 Xuanwumen West St, Xicheng District
Beijing 10053
Email: gengliang@chinamobile.com
8. Acknowledges
We would like to thank Rob Wilton, Thomas Graf, Andy Bierman, Michael
Richardson, Henk Birkholz, Chong Feng, Adrian Farrel for valuable
review on this document, special thanks to Thomas and Michael for
organizing the discussion on several relevant drafts and reach the
common understanding on the concept and ideas. Thanks Michael for
providing CHIP/Matter WIFI statistics reference.
9. References
9.1. Normative References
[I-D.ietf-netmod-rfc6991-bis]
Schönwälder, J., "Common YANG Data Types", Work in
Progress, Internet-Draft, draft-ietf-netmod-rfc6991-bis-
15, 23 January 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-netmod-
rfc6991-bis-15>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC3198] Westerinen, A., Schnizlein, J., Strassner, J., Scherling,
M., Quinn, B., Herzog, S., Huynh, A., Carlson, M., Perry,
J., and S. Waldbusser, "Terminology for Policy-Based
Management", RFC 3198, DOI 10.17487/RFC3198, November
2001, <https://www.rfc-editor.org/info/rfc3198>.
[RFC5277] Chisholm, S. and H. Trevino, "NETCONF Event
Notifications", RFC 5277, DOI 10.17487/RFC5277, July 2008,
<https://www.rfc-editor.org/info/rfc5277>.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<https://www.rfc-editor.org/info/rfc6241>.
[RFC6242] Wasserman, M., "Using the NETCONF Protocol over Secure
Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,
<https://www.rfc-editor.org/info/rfc6242>.
[RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<https://www.rfc-editor.org/info/rfc7950>.
[RFC8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
<https://www.rfc-editor.org/info/rfc8040>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8341] Bierman, A. and M. Bjorklund, "Network Configuration
Access Control Model", STD 91, RFC 8341,
DOI 10.17487/RFC8341, March 2018,
<https://www.rfc-editor.org/info/rfc8341>.
[RFC8342] Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
and R. Wilton, "Network Management Datastore Architecture
(NMDA)", RFC 8342, DOI 10.17487/RFC8342, March 2018,
<https://www.rfc-editor.org/info/rfc8342>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
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[RFC8639] Voit, E., Clemm, A., Gonzalez Prieto, A., Nilsen-Nygaard,
E., and A. Tripathy, "Subscription to YANG Notifications",
RFC 8639, DOI 10.17487/RFC8639, September 2019,
<https://www.rfc-editor.org/info/rfc8639>.
[RFC8641] Clemm, A. and E. Voit, "Subscription to YANG Notifications
for Datastore Updates", RFC 8641, DOI 10.17487/RFC8641,
September 2019, <https://www.rfc-editor.org/info/rfc8641>.
[RFC9196] Lengyel, B., Clemm, A., and B. Claise, "YANG Modules
Describing Capabilities for Systems and Datastore Update
Notifications", RFC 9196, DOI 10.17487/RFC9196, February
2022, <https://www.rfc-editor.org/info/rfc9196>.
9.2. Informative References
[CHIP] CSA, "Connected Home over IP Specification", April 2021.
[IEEE754-2008]
IEEE, "IEEE Standard for Floating-Point Arithmetic", DOI
10.1109/IEEESTD.2008.4610935, 2008,
<http://standards.ieee.org/findstds/
standard/754-2008.html>.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
DOI 10.17487/RFC3688, January 2004,
<https://www.rfc-editor.org/info/rfc3688>.
[RFC6020] Bjorklund, M., Ed., "YANG - A Data Modeling Language for
the Network Configuration Protocol (NETCONF)", RFC 6020,
DOI 10.17487/RFC6020, October 2010,
<https://www.rfc-editor.org/info/rfc6020>.
[RFC8340] Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
<https://www.rfc-editor.org/info/rfc8340>.
[XPATH1.0] W3C, "https://www.w3.org/TR/1999/REC-xpath-19991116/", 11
November 1999.
Appendix A. Example YANG Module
This section presents an example YANG module so that Appendix B can
give examples of how the YANG module defined in Section 4 is used to
perform adaptive subscription. The example YANG module used in this
section represents a Wi-Fi Network Diagnostics data specified in
[CHIP] which can be used by a Node to assist a user or Administrative
Node in diagnosing potential problems.
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YANG tree diagram for the "example-wifi-network-diagnostic" module:
module: example-wifi-network-diagnostic
+--ro server
| +--ro bssid? yang:mac-address
| +--ro security-type? enumeration
| +--ro wifi-version? enumeration
| +--ro channel-num? int8
| +--ro rssi? int8
| +--ro beacon-lost-count? int8
| +--ro beacon-rx-count? int8
| +--ro packet-multicast-rx-count? int8
| +--ro packet-multicast-tx-count? int8
| +--ro packet-unicast-rx-count? int8
| +--ro packet-unicast-tx-count? int8
| +--ro current-max-rate? int8
| +--ro overrun-count? int8
+--ro events
+--ro event* [name]
+--ro name string
+--ro disconnection? enumeration
+--ro association-failure? enumeration
+--ro connection-status? enumeration
A.1. "example-wifi-network-diagnostic" YANG Module
module example-wifi-network-diagnostic {
yang-version 1;
namespace "http://example.com/yang/wifi-network-diagnostic";
prefix wnd;
import ietf-yang-types {
prefix yang;
}
container server {
config false;
description
"Configuration of the WiFi Server logical entity.";
leaf bssid {
type yang:mac-address;
description
"The MAC address of a wireless access point.";
}
leaf security-type {
type enumeration {
enum unspecified {
value 0;
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}
enum none {
value 1;
}
enum wep {
value 2;
}
enum wpa {
value 3;
}
enum wpa2 {
value 4;
}
enum wpa3 {
value 5;
}
}
description
"The type of Wi-Fi security used. A value of 0
indicate that the interface is not currently
configured or operational.";
}
leaf wifi-version {
type enumeration {
enum 80211a {
value 0;
}
enum 80211b {
value 1;
}
enum 80211g {
value 2;
}
enum 80211n {
value 3;
}
enum 80211ac {
value 4;
}
enum 80211ax {
value 5;
}
}
description
"The highest 802.11 standard version usable
by the Node.";
}
leaf channel-num {
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type int8;
description
"The channel that Wi-Fi communication is currently
operating on. A value of 0indicates that the interface
is not currently configured or operational.";
}
leaf rssi {
type int8;
description
"The RSSI of the Node's Wi-Fi radio in dBm.";
}
leaf beacon-lost-count {
type int8;
description
"The count of the number of missed beacons the
Node has detected.";
}
leaf beacon-rx-count {
type int8;
description
"The count of the number of received beacons. The
total number of expected beacons that could have been
received during the interval since association SHOULD
match the sum of BeaconRxCount and BeaconLostCount. ";
}
leaf packet-multicast-rx-count {
type int8;
description
"The number of multicast packets received by
the Node.";
}
leaf packet-multicast-tx-count {
type int8;
description
"The number of multicast packets transmitted by
the Node.";
}
leaf packet-unicast-rx-count {
type int8;
description
"The number of multicast packets received by
the Node.";
}
leaf packet-unicast-tx-count {
type int8;
description
"The number of multicast packets transmitted by
the Node.";
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}
leaf current-max-rate {
type int8;
description
"The current maximum PHY rate of transfer of
data in bytes-per-second.";
}
leaf overrun-count {
type int8;
description
"The number of packets dropped either at ingress or
egress, due to lack of buffer memory to retain all
packets on the ethernet network interface. The
OverrunCount attribute SHALL be reset to 0 upon a
reboot of the Node..";
}
}
container events {
config false;
description
"Configuration of WIFI Network Diagnostic events.";
list event {
key "name";
description
"The list of event sources configured on the
server.";
leaf name {
type string;
description
"The unique name of an event source.";
}
leaf disconnection {
type enumeration {
enum de-authenticated {
value 1;
}
enum dis-association {
value 2;
}
}
description
"A Node's Wi-Fi connection has been disconnected as a
result of de-authenticated or dis-association and
indicates the reason.";
}
leaf association-failure {
type enumeration {
enum unknown {
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value 0;
}
enum association-failed {
value 1;
}
enum authentication-failed {
value 2;
}
enum ssid-not-found {
value 3;
}
}
description
"A Node has attempted to connect, or reconnect, to
a Wi-Fi access point, but is unable to successfully
associate or authenticate, after exhausting all
internal retries of its supplicant.";
}
leaf connection-status {
type enumeration {
enum connected {
value 1;
}
enum notconnected {
value 2;
}
}
description
"A Node's connection status to a Wi-Fi network has
changed. Connected, in this context, SHALL mean that
a Node acting as a Wi-Fi station is successfully
associated to a Wi-Fi Access Point.";
}
}
}
}
Appendix B. Adaptive Subscription and Notification Example
The examples within this document use the normative YANG module
"ietf-adapt-subscription" defined in Section 4 and the non-normative
example YANG module "example-wifi-network-diagnostic" defined in
Appendix A.1.
This section shows some typical adaptive subscription and
notification message exchanges.
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B.1. "edit-config" Example
The client configures adaptive subscription policy parameters on the
server. The adaptive subscription configuration parameters require
the server to support two update intervals (i.e., 5 seconds, 60
seconds) and report updates every 60 seconds if the rssi value is
greater than or equal to -65dB; If the rssi value is less than -65dB,
switch to 5 seconds period value to report updates.
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<rpc message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<edit-config>
<target>
<running/>
</target>
<config>
<subscriptions
xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications"
xmlns:yp="urn:ietf:params:xml:ns:yang:ietf-yang-push">
<subscription>
<id>1011</id>
<yp:datastore
xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">
ds:operational
</yp:datastore>
<yp:datastore-xpath-filter
xmlns:wnd="http://example.com/yang/wifi-network-diagnostic">
/wnd:server
</yp:datastore-xpath-filter>
<as:adaptive-periods
xmlns:as="urn:ietf:params:xml:ns:yang:ietf-adapt-subscription">
<as:adaptive-period>
<as:name>adaptive-period-1</as:name>
<as:xpath-eval-criterion>
/wnd:server/wnd:rssi<-65
</as:xpath-eval-criterion>
<as:period>5</as:period>
</as:adaptive-period>
<as:adaptive-period>
<as:name>adaptive-period-2</as:name>
<as:xpath-eval-criterion>
/wnd:server/wnd:rssi>=-65
</as:xpath-eval-criterion>
<as:period>60</as:period>
</as:adaptive-period>
</as:adaptive-periods>
<receivers>
<receiver>
<name>host.example.com</name>
</receiver>
</receivers>
</subscription>
</subscriptions>
</config>
</edit-config>
</rpc>
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B.2. Create Adaptive Subscription Example
The subscriber sends an "establish-subscription" RPC with the
parameters listed in to request the creation of an adaptive
subscription. The adaptive subscription configuration parameters
require the server to report updates every 5 seconds if the rssi
value is less than -65dB; If the rssi value is greater than or equal
to -65dB, switch to 60 seconds period value. (Section 2)
<netconf:rpc message-id="101"
xmlns:netconf="urn:ietf:params:xml:ns:netconf:base:1.0">
<establish-subscription
xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications"
xmlns:yp="urn:ietf:params:xml:ns:yang:ietf-yang-push">
<yp:datastore
xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">
ds:operational
</yp:datastore>
<yp:datastore-xpath-filter
xmlns:wnd="http://example.com/yang/wifi-network-diagnostic">
/wnd:server
</yp:datastore-xpath-filter>
<as:adaptive-periods
xmlns:as="urn:ietf:params:xml:ns:yang:ietf-adapt-subscription">
<as:adaptive-period>
<as:name>adaptive-period-1</as:name>
<as:xpath-eval-criterion>
/wnd:server/wnd:rssi<-65
</as:xpath-eval-criterion>
<as:period>5</as:period>
</as:adaptive-period>
<as:adaptive-period>
<as:name>adaptive-period-2</as:name>
<as:xpath-eval-criterion>
/wnd:server/wnd:rssi>=-65
</as:xpath-eval-criterion>
<as:period>60</as:period>
</as:adaptive-period>
</as:adaptive-periods>
</establish-subscription>
</netconf:rpc>
B.3. "xpath-evaluation-unsupported" error response example
If the subscriber has authorization to establish the subscription
with a server, but the server had not been able to fully satisfy the
request from the subscriber, the server should send an RPC error
response.
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For instance, if the XPATH 1.0 syntax against the targeted data node
defined in "xpath-eval-criterion" is not supported by the server' s
implementation, the server returns a reply indicating a failure. The
following <rpc-reply> illustrates an example:
<?xml version="1.0" encoding="utf-8"?>
<rpc-reply message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<rpc-error>
<error-type>application</error-type>
<error-tag>invalid-value</error-tag>
<error-severity>error</error-severity>
<error-app-tag>
ietf-adapt-subscription:xpath-evaluation-unsupported
</error-app-tag>
<error-path
xmlns:wnd="http://example.com/yang/wifi-network-diagnostic">
/wnd:server/wnd:rssi
</error-path>
</rpc-error>
</rpc-reply>
Since adaptive subscription allows a server to be configured with
multiple different period intervals and corresponding XPath
evaluation criteria to trigger update interval switch in the server,
it may be possible for the server to return multiple <rpc-error>
elements with "xpath-evaluation-unsupported" failure specified by
different error paths. The subscriber can use this information in
future attempts to establish a subscription.
B.4. "adaptive-period-update" notification example
Upon the server switches from the update interval 5 seconds to the
new update interval 60 seconds, before sending event records to
receivers, the "adaptive-period-update" notification should be
generated and sent to the receivers to inform the receivers that the
update interval value is switched to the new value.
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<notification
xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0"
xmlns:yp="urn:ietf:params:xml:ns:yang:ietf-yang-push">
<eventTime>2016-11-21T13:51:00Z</eventTime>
<adaptive-period-update
xmlns="urn:ietf:params:xml:ns:yang:ietf-adapt-subscription">
<id>1011</id>
<period>60</period>
<yp:datastore
xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">
ds:operational
</yp:datastore>
<yp:datastore-xpath-filter
xmlns:wnd="http://example.com/yang/wifi-network-diagnostic">
/wnd:server
</yp:datastore-xpath-filter>
</adaptive-period-update>
</notification>
Appendix C. Changes between Revisions
This section is to be removed before publishing as an RFC.
v03 -v04
* Add CPU utilization case in introduction section;
* Change the Xpath expression parameter "xpath-external-eval" to
"xpath-eval-criterion";
* Some tweaks to the XPath Complexity Evaluation section;
* Editorial updates (e.g., fix long lines);
v02 -v03
* Augment "modify-subscription", "subscription-started",
"subscription-modified" to also support adaptive-subscription
parameters;
* Replace the "anchor-time" parameter defined in adaptive-period-
update notification with "period-update-time";
* Fix the YANG module and XML snippet errors in Appendix sections.
v01 -v02
* Editorial changes to improve readability
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* Clarify that "period" and "xpath-external-eval" parameters must be
co-exist so that the period can be switched based on trigger
conditions indicated by "xpath-external-eval"
v00 -v01
* Clarify what if multiple Xpath condition expressions conflict with
each other during the lifecycle of an adaptive subscription
* Clarify that existing RPC errors defined in RFC 8639 and 8641 are
still supported by this document
* Refine the YANG module: add contact information, fix IETF Trust
Copyright statement, fix yanglint validation error
v09 -v10
* Change the draft intended status to "experimental"
* Problem statement refinement
v08 -v09
* Define two new RPC errors to report when adaptive subscription
unsupported or multiple XPath criteria conflict.
* Remove the "watermark" parameter.
* Add clarification about how to evaluate the XPath expression
defined in "xpath-external-eval".
* Add clarification about how to compare a targeted data object in a
specific list entry.
v07 -v08
* Define a new RPC error to report when an XPath syntax defined in
"xpath-external-eval" is unsupported by a server.
* Add a new example showing how the RPC error being returned by a
publisher.
* The usage examples fixed in the Appendix.
* Grammatical errors correction(missing articles, plurality
mismatches, etc).
v06 -v07
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* The usage examples typo fixed in the Appendix.
* Add reference to RFC7950 XPATH Evaluation section and XPATH 1.0
* Clarify the definitions of 'xpath-external-eval' and 'selection-
filter' by reusing XPATH Evaluation rules in RFC7950.
* Add a new terminology "adaptive subscription".
* Add one section to discuss Arbitrary XPath Complexity.
v05 -v06
* Replace example-wifi-mac module with example-wifi-network-
diagnostic using WIFI statistics specified in CHIP specification.
* Update adaptive subscription Example to align with WIFI example
module change.
* Add one more reference to CHIP Specification.
v04 -v05
* Remove "modify-subscption" RPC usage.
* Module update to fix the nits.
* Update adaptive subscription Example.
* Other Editorial changes.
v03 - v04
* Add missing subtrees and data nodes in the security section;
* Change "adaptive-update" notification into "adaptive-period-
update" notification;
* Other Editorial changes.
v02 - v03
* Clarify the difference between low priority telemetry data
dropping and collection rate switching in the introduction
section;
Wu, et al. Expires 14 June 2024 [Page 33]
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* Update the abstract and introduction section to focus on
collection rate switching in the server without interaction with
the remote client;
* Format usage example and change ssid into rssi in the appendix;
* Use boilerplate and reuse the terms in the terminology section.
Authors' Addresses
Qin Wu
Huawei
101 Software Avenue, Yuhua District
Nanjing
Jiangsu, 210012
China
Email: bill.wu@huawei.com
Wei Song
Huawei
101 Software Avenue, Yuhua District
Nanjing
Jiangsu, 210012
China
Email: songwei80@huawei.com
Peng Liu
China Mobile
32 Xuanwumen West St, Xicheng District
Beijing
Email: liupengyjy@chinamobile.com
Qiufang Ma
Huawei
101 Software Avenue, Yuhua District
Nanjing
Jiangsu, 210012
China
Email: maqiufang1@huawei.com
Wei Wang
China Telecom
32 Xuanwumen West St, Xicheng District
Beijing
Wu, et al. Expires 14 June 2024 [Page 34]
Internet-Draft Adaptive Subscription December 2023
Email: wangw36@chinatelecom.cn
Zhixiong Niu
Microsoft
Email: Zhixiong.Niu@microsoft.com
Wu, et al. Expires 14 June 2024 [Page 35]
